1. 1. Sixteen multiple forms of soluble esterase activity have been resolved from pig tissue extracts. 2. 2. Duck tissue esterases have also been shown to exist in multiple forms and exhibit nineteen bands of activity. 3. 3. By means of substrate and inhibitor studies, these heteromorphs have been characterized into four main classes: carboxylesterase, cholinesterase, arylesterase and acetylesterase. {A table is presented} 4. 4. Further differentation of the multiple forms in some of the classes has been achieved from an investigation into the development and physicochemical properties of these enzymes.